Prior to my current center-channel speaker (Hales Revelation Center) I was using an Infinity SM-Video made in the late '90's paired with Infinity SM65 studio monitors. It had dual 6.5" woofs which flanked a single 1" polycell tweet in a fairly big ported box. While certainly not as refined as the Hales, it definately delivered crystal-clear dialog. It's been discontinued but could probably be found used for well under $300(original retail was around $350). NHT has some nice offerings in your price range as well, but I don't know if either would match your current speakers in timbre, etc.
Unlike traditional upright speakers, many horizontal oriented center channel speakers suffer from moderate to severe off-axis lobing which will muddy up the midrange. That's one reason they have earned a bad reputation for poor dialog intelligability. The frequency response and tonal quality can really suffer in as little as 10 or 15 degrees off-axis with some speakers. The more advanced designs may utilize a midrange driver and/or crossover tweaks, etc. to help minimize the lobing. You may have to invest a little more than your budget to get to where you want to be..
Unlike traditional upright speakers, many horizontal oriented center channel speakers suffer from moderate to severe off-axis lobing which will muddy up the midrange. That's one reason they have earned a bad reputation for poor dialog intelligability. The frequency response and tonal quality can really suffer in as little as 10 or 15 degrees off-axis with some speakers. The more advanced designs may utilize a midrange driver and/or crossover tweaks, etc. to help minimize the lobing. You may have to invest a little more than your budget to get to where you want to be..